Landis provides an amazing amount of detail covering the most important streams across the state. Inspired by the streams and their surrounding landscapes, he wrote this 1st edition of this Pennsylvania fly-fishing guidebook in 1991 at a time when there were very few books of it's type.
His book was one of the inspirations for Paflyfish.com and I personally pack his book with me as I trek out on my fly-fishing jaunts.
The reprinted 3rd edition (no changes) can be found at many local fly shops and online Trout Streams of Pennsylvania: An Angler's Guide, Third Edition
An interview with Dwight on Paflyfish can be found here.